Exposure controlled corona device

ABSTRACT

A corotron is disclosed herein in which the surrounding shield is coated with a photoconductive insulator. The extent to which the shield is grounded is controlled by the extent to which it is subjected to illumination. When the shield is illuminated and fully grounded, the corotron operates in the manner known to the prior art. When the corotron is in darkness and therefore not grounded because of the photoconductive insulator, charge accumulates on the shield causing the corona wire to stop emitting.

United States Patent North June 11, 1974 Primary Examiner-James W.Lawrence Assistant Examiner-C. E. Church [76] Inventor: gz m g g" 2 3;rset Attorney, Agent, or Firm-Robert J. Bird [22] Filed: Nov. 15, 197257 ABSTRACT [21] Appl. No.: 306,766 A corotron is disclosed herein inwhich the surrounding shield is coated with a photoconductive insulator.The extent to which the shield is grounded is coni g ggfig trolled bythe extent to which it is subjected to illumi- [58] Fieid 317/262 Anation. When the shield is illuminated and fully grounded, the corotronoperates in the manner known to the prior art. When the corotron is indarkness and [56] References Cited 1 therefore not grounded because ofthe photoconduc- UNITED STATES PATENTS tive insulator, chargeaccumulates on the shield caus- 2,868,989 1/1959 Haacke... 250/495 ZCing the corona wire to top emitting, 3,220,324 11/1965 Snelling..250/495 ZC 3,711,710 1/1973 Wright 250/495 zc 2 Chums, 2 Drawlng FlguresL. /6' O /Z 3 1 EXPOSURE CONTROLLED CORONA DEVICE BACKGROUND OF THEINVENTION The present invention relates to the field of xerography andin particular to an improved corona generating device for imposingelectrostatic charge on a xerographic surface.

In xerography, one known method by which a photoconductive surface, orphotoreceptor, is sensitized is by the use of ions generated in a coronadischarge. To effect the corona discharge, one or more corona wires ismaintained at a high potential andthe resultant electric field strengthat the surface of the wire causes air molecules to become ionized. Theseions are drawn to the xerographic plate by thedifference' in potentialexisting between wire and plate. The build-up of these ions on the platecreates a potential across the plate.

In general, a grounded shield is used in association with the coronawire so as to control and direct its discharge onto the photoreceptor.

Reference is hereby made to U. S. Pat. No. 2,836,725, issued to RobertG.Vyverberg, wherein corona generating devices of the type brieflydescribed above are more fully disclosed.

One characteristic of corona generating devices, or corotrons as theyhave become known, is that the corona-emitting wire is maintainedat ahigh voltage. If it is desired to switch this voltage on and off,elaborate and expensive switching apparatus would be required to handlethis high voltage.

SUMMARY OF THE INVENTION It is an object of the'present invention toprovide a shield for a corona generating apparatus, which shield iscapable of controllingthe level of emission from the corona wire.

Another object is to provide an optical means for switching a highvoltage corona wire.

Briefly, this invention is practiced in one form by a grounded corotronshield which is lined with a photoconductive insulator. Whenilluminated, the photoconductive material becomes conductive and theshield is fully grounded. When not illuminated, the photoconductor is aninsulator and the corona shield collects charge from the corona wire.When'the shield becomes charged to a potential near that of the wire,the wire ceases to emit andthus the corotron is switched off.

For a better understanding of this invention, reference is made to thefollowing detailed description given in connection with the accompanyingdrawing.

FIG. 1 is a somewhat schematic representation of a corotron shieldmember, all disclosed and adjacent shield in operative position relativeto a charge plate.

FIG. 2 is a fragmentary detail showing the structure of a corotronshield according to this invention.

DESCRIPTION Referring now to FIG. I, there is shown the generalenvironment in which the present invention is used. It includes acorotron generally indicated at 2 disposed relative to a xerographicphotoreceptor 4. As is well known, the corotron 2 acts to ionize theadjacent air and the resulting ions flow in a corona current to thecorotron shield 6 and to the photoreceptor 4. Shield 6 is partlycomposed of a conductive material and is grounded. Photoreceptor 4includes a conductive grounded plate 8 and a coating of photoconductiveinsulation 10. A corona wire 12 disposed within the shield 6 ismaintained at a high voltage for the purpose of effecting the desiredcorona current. The foregoing is a brief summary of the prior art.

Referring now to FIG. 2, the structure of the corotron shield 6 of thepresent invention is shown in greater detail. The shield is shown onlyfragmentarily since its geometrical configuration is not material tothis invention. Shield 6 is composed of a transparent outer material 14witha conductive layer 16 on its inner surface. The conductive layer 16is connected to ground. A photoconductive layer 18 is in turn applied tothe conductive layer 16. A light source is schematically represented at20 and positioned to illuminate the corotron shield 6.

One suitable material for the outer transparent material is NESA glass.The conductive layer 16 is either a transparent layer orsemi-transparent, so as to permit illumination from source 20 to reachthe photoconductive layer 18.

The operation of this corotron structure is as follows: When the shieldis illuminated, the photoconductive layer 18 is conductive and thecorotron shield 6 is therefore conductive and grounded in the usualmanner. In the absence of light, the photoconductive layer 18 is aninsulator so that charges emitting from the corotron wire 12 accumulateon the surface of layer 18. When the potential on the layer 18, that isto say on the inner surface of the corotron shield 6, reaches that ofthe corotron wire 12, the wire ceases to emit and the corotron isthereby switched off.

The intensity of light from source 20 determines the degree to which thephotoconductive layer 18 is conductive (or insulative) and thusdetermines the degree to which the corotron emits. Additionally, thecorotron and shield may be arranged so that the wire-to-shield spacingis less than the wire-to-plate spacing, permitting a greater measure ofcontrol by the shield over the corotron emission.

By the simple switching on and off of a light source, or the control ofits intensity, the corotron of this invention is controllable as tocurrent output level. This corotron shield obviates the necessity forexpensive or intricate switching gear otherwise required for use withhigh voltages.

While the light. source 20 has been schematically represented on theside of the shield 6 opposite that of the corona wire 12, it may ofcourse be located on the same side of the shield.

The foregoing description of an embodiment of this invention is given ofway of illustration and not of limitation. The concept .andscope of theinvention are limited only by the following claims and equivalentsthereof which may occur to others skilled in the art.

What is claimed is:

l. A corona generating device including a corona emission electrode anda shield electrode operatively disposed relative to said emissionelectrode, said shield electrode including an electrically conductivemember which is connected to ground anda photoconductive insulativemember electrically connected to said conductive member and disposedbetween said conductive member and said emission electrode, and

optical communication with said photoconductive insulative layer,

whereby corona charge emitting from said emission electrode to saidshield electrode flows to ground when said insulative layer isilluminated and accumulates on said shield when said insulative layer isnot illuminated.

1. A corona generating device including a corona emission electrode anda shield electrode operatively disposed relative to said emissionelectrode, said shield electrode including an electrically conductivemember which is connected to ground and a photoconductive insulativemember electrically connected to said conductive member and disposedbetween said conductive member and said emission electrode, and means toilluminate said photoconductive insulative member to render the sameconductive.
 2. A corona generating device including a corona emissionelectrode and a shield electrode partially surrounding said emissionelectrode, said shield electrode including a grounded electricallyconductive layer and a photoconductive insulative layer disposed on andelectrically connected to said conductive layer on theemission-electrode side thereof, and a light source in opticalcommunication with said photoconductive insulative layer, whereby coronacharge emitting from said emission electrode to said shield electrodeflows to ground when said insulative layer is illuminated andaccumulAtes on said shield when said insulative layer is notilluminated.